The present invention relates to a transfer medium for transferring a protective layer. More particularly, the invention relates to a transfer medium for transferring a protective layer by a thermal transfer method to impart excellent durability to plastics articles in the form of sheet or molded article, images formed on various receptors, etc.
Heretofore, gradation images such as color pictures or monotone images such as letters and symbols have been formed by an electrophotographic method, an ink jet printing method, a thermal transfer method or the like. The thus formed images are frequently required to have scratch resistance or solvent resistance depending upon the type of their uses. There is a method for imparting durability to an image wherein the image is formed on a receptor and a protective layer is formed on the image by solid-printing using a thermal transfer medium (this process is hereinafter referred to as “overprinting”). According to this method, the image can be protected in an easy manner at low cost.
In the overprinting method, the durability of the image can be improved by curing the protective layer. Generally the curing of the protective layer is conducted by using heat or ionizing radiation (see patent reference 1). In the case of curing with heat, the protective layer is previously cured in a state wherein it is present in a thermal transfer medium because the heat curing usually needs a long period time and the heat curing of the protective layer after being transferred results in degradation in easiness of the system. Further the heat curing after the transfer sometimes causes heat deformation of image bearing articles.
In the case of curing using ionizing radiation, the protective layer may be cured before or after the transfer from the viewpoint of easiness of the system. However, when the protective layer is previously cured in a state wherein it is present in the thermal transfer medium to improve the durability as described above, the heat-meltability of the protective layer is markedly degraded resulting in poor selective transferability of the protective layer. The term “selective transferability”, as used herein, refers to the property that only a heated portion of a layer is transferred but unheated portions in the periphery of the heated portion are not transferred. As a result, in the transfer of the protective layer at the edge portion of character, there sometimes occurs a phenomenon that a heated portion of the protective layer is transferred together with an unheated portion adjacent to the heated portion (this phenomenon is referred to as “planar peeling”). In this case, it is necessary to make the protective layer thinner to obtain a satisfactory selective transferability and the thickness of the protective layer is limited to a relatively smaller one, resulting in failure to achieve high level durability.
In the case of curing the protective layer by irradiation with an ionizing radiation after the transfer, there are no such problems as mentioned above but there is another problem as follows: Generally ionizing radiation-curable resins have tackiness before curing. A transfer medium wherein a layer of such an ionizing radiation-curable resin is formed on a support cannot be wound into a roll like ink ribbons. It is necessary to apply a separator onto the ionizing radiation-curable resin layer in order to wind the transfer medium into a roll (see patent reference 2), resulting in high cost. Further, it is difficult to form an adhesive layer onto the ionizing radiation-curable resin layer due to the tackiness of the ionizing radiation-curable resin layer. Consequently, the conventional protective layer transfer medium can be only used to apply a protective layer onto a receptor to which the ionizing radiation-curable resin layer shows good adhesion and lacks in application flexibility.    Patent reference 1: JP A 5-330259    Patent reference 2: JP A 5-57235
It is an object of the present invention to provide a transfer medium for transferring a protective layer, which protective layer has excellent selective transferability upon thermally transferring it onto an image formed according to a variety of printing methods to protect the image and does not have tackiness before curing.
This and other objects of the present invention will become apparent from the description hereinafter.